1. Field of the Invention
The present invention relates to a life determination device for a DC capacitor connected to a DC side of a rectifier.
2. Description of the Related Art
In a motor control device for driving a motor of machine tools, an industrial machine, a forge rolling machine, an injection machine, or various robots, an AC power inputted from an AC power supply side is converted to a DC power, which in turn is converted to an AC power, and the converted AC power is used as driving power for a motor provided for each drive shaft.
FIG. 4 is a view illustrating the configuration of a general motor control device. The motor control device 100 includes a rectifier 101 that converts AC power from a commercial three-phase AC power supply 103 (hereinafter referred to simply as “power supply”) to DC power, an inverter 102 that converts the DC power outputted from the rectifier 101 to AC power of a desired frequency to be supplied as driving power for a motor 104 or converts AC power regenerated from the motor 104 to DC power, and controls the speed or rotor position of the motor 104 connected to the AC side of the inverter 102. The rectifier 101 and the inverter 102 are connected via a direct current link (DC link. In the DC link is provided a DC capacitor (DC link capacitor) 105.
The DC capacitor 105 is generally known as a component having a limited life time of which electrostatic capacitance (hereinafter, referred to simply as “capacitance”) is decreased by repeated charging and discharging. When the capacitance of the DC capacitor 105 is decreased, a problem arises that the ripple current flowing through the DC link is increased which results in increased fluctuations in the DC voltage. Thus, it is important to accurately grasp the capacitance of the DC capacitor 105 in order to determine whether the end of the life of the DC capacitor is reached. The DC capacitor 105, which is determined that the end of its life is reached, needs to be replaced.
As a technique for estimating the electrostatic capacitance of a DC capacitor provided in a DC link of a rectifier, there is a device for determining the life of a DC capacitor by measuring the charging time and the charging voltage when the DC capacitor is charged with a constant current, and comparing the result of the measurement with preset characteristics of the charging time and the charging voltage, as described in Japanese Unexamined Patent Publication No. 2008-17613, for example. FIG. 5 is a schematic view for explaining the invention described in Japanese Unexamined Patent Publication No. 2008-17613. Meanwhile, in FIG. 5, there are omitted the illustrations of an inverter connected to the rectifier 101 via the DC capacitor in the DC link and a motor driven by the inverter. In this device, there are provided switches SW1 and SW2, an inductor 106, and a DC capacitor charging control circuit 110 for charging the DC capacitor with a constant current I resulting from conversion by the rectifier 101 of an AC current from an AC power supply 103 to a DC current, and the measured charging time and charging voltage are compared with preset characteristics of the charging time and the charging voltage by a DC capacitance estimating circuit 111, thereby determining the life of the DC capacitor.
Further, there is a device that determines the life of a DC capacitor by charging and discharging the DC capacitor in a DC link by a charging circuit and a discharging circuit without using a complex charging control circuit, and measuring and comparing the charging time and the discharging time, as described in Japanese Unexamined Patent Publication No. 2002-98725, for example. FIG. 6 is a schematic view for explaining the invention described in Japanese Unexamined Patent Publication No. 2002-98725. Meanwhile, in FIG. 6, there are omitted illustrations of an inverter connected to a rectifier 101 via a DC capacitor in a DC link and a motor driven by the inverter. In this device, there are provided switches SW1 and SW2, a charging resistor 107, a discharging resistor 108, an inductor 106, and a DC capacitor charging and discharging control circuit 112 for charging and discharging a DC capacitor with a current I resulting from conversion by the rectifier 101 of an AC current from an AC power supply 103 to a DC current; the DC capacitor 105 is charged and discharged by these components; then the charging time and the discharging time are measured, and the measured charging time and discharging time are compared by a DC capacitor capacitance estimating circuit 113; and in this manner the life of the DC capacitor 105 is determined.
Further, there is a device that determines the life of a DC capacitor using a time-integral value of the charging current when the DC capacitor is charged and a voltage value of the DC capacitor, without being influenced by fluctuations in an AC power supply when the DC capacitor in a DC link is charged and changes in time constants of a charging circuit and a discharging circuit, as described in Japanese Unexamined Patent Publication No. 2000-152643, for example. FIG. 7 is a schematic view for explaining the invention described in Japanese Unexamined Patent Publication No. 2000-152643. Meanwhile, in FIG. 7, there are omitted the illustrations of an inverter connected to a rectifier 101 via the DC capacitor in the DC link and a motor driven by the inverter. In this device, there are provided a switch SW1, a charging resistor 107, and a DC capacitor charging control circuit 115 for charging the DC capacitor with a current I resulting from conversion by the rectifier 101 of an AC current from the AC power supply 103 to a DC current. In this regard, a charging current integrating circuit 116 time-integrates the charging current I during the charging period of the DC capacitor 105, and a DC capacitor capacitance estimating circuit 117 calculates an estimated capacitance C of the DC capacitor 105 from a resultant current integration value and a voltage V of the DC capacitor 105 based on equation (1) as follows:C=∫Idt÷V  (1)
Further, it is determined by a life determination circuit 118 whether the capacitance of the DC capacitor 105 is decreased as compared with the estimated capacitance C and a reference capacitance set by a reference value setting unit 119.
When the capacitance of the DC capacitor is decreased by repeated charging and discharging of the DC capacitor provided in the DC link at the DC output side of the rectifier as described above, a problem arises that the ripple current flowing through the DC link is increased so that fluctuations in the DC voltage become large; hence, it is important to accurately measure the capacitance of the DC capacitor. Failure to accurately measure the capacitance of the DC capacitor might lead to a possibility that the timing for replacing the DC capacitor is missed so that a large ripple current or DC voltage fluctuation is caused to occur in the DC link. Further, it might, alternatively, lead to a possibility that the DC capacitor, which has not yet reached the end of its life, is replaced unnecessarily early.
In the invention described in Japanese Unexamined Patent Publication No, 2008-17613, for example, a DC capacitor is charged with a constant current resulting from convention by a rectifier of an AC current from an AC power supply to a DC current, and hence there is a problem such that a plurality of switches, an inductor, and a DC charging control circuit must be provided so that the circuit arrangement is complicated. Further, there is a problem such that in order to apply such invention to an existing motor control device including a rectifier and an inverter connected to the rectifier via a DC link, the existing circuit configuration must be modified significantly.
Further, according to the invention described in Japanese Unexamined Patent Publication No. 2002-98725, for example, the DC capacitor charging control circuit does not become so complex, but since not only a charging circuit but also a discharging circuit must be provided, there is a problem such that in order to apply such invention to an existing motor control device, the circuit configuration must be modified significantly.
Further, according to the invention described in Japanese Unexamined Patent Publication No. 2002-98725, when the voltage of the AC power supply 103 is changed, the charging time is changed so that it is not possible to accurately determine the life of the DC capacitor. Further, when the time constants of the charging circuit and the discharging circuit are changed, too, the charging time and the discharging time are changed so that it is not possible to accurately determine the life of the DC capacitor. FIG. 8 is a view for explaining relationships between charging resistance and charging time in the invention described in Japanese Unexamiend Patent Publication No. 2002-98725. When the resistance value of the charging resistor 107 is changed, the charging time is also changed. For example, when the resistance value of the charging resistor 107 is decreased to be smaller than a normal value, the charging current becomes larger than when the resistance value is the normal value, so that the charging time t1 becomes shorter than a normal charging time t. Further, when the resistance value of the charging resistor 107 is increased to be larger than the normal value, the charging current becomes smaller than when the resistance value is the normal value, so that the charging time t2 becomes shorter than the normal charging time t. FIG. 9 is a view for explaining relationships between discharging resistance and discharging time in the invention described in Japanese Unexamined Patent Publication No. 2002-98725. When the value of the discharging resistor 108 is changed, the discharging time is also changed. For example, when the resistance value of the discharging resistor 108 is increased to be larger than the normal value, the amount of electric charges consumed by the discharging resistor 108 becomes smaller than when the resistance value is the normal value, so that the discharging time t4 becomes longer than the normal discharging time t. Further, when the resistance value of the discharging resistor 108 is decreased to be smaller than the normal value, the amount of electric charges consumed by the discharging resistor 108 becomes larger than when the resistance value is the normal value, so that the discharging time t3 becomes longer than the normal discharging time t. In this manner, according to the invention described in Japanese Unexamined Patent Publication No. 2002-98725, when the voltage of the AC power supply serving as a supply source of the charging current is changed, the charging time is changed, and when the time constants of the charging circuit and the discharging circuit are changed, the charging time and the discharging time are changed; thus, there is a problem that it is not possible to accurately determine the life of the DC capacitor.
Further, according to the invention described in Japanese Unexamined Patent Publication No. 2000-152643, for example, it is possible to determine the life of the DC capacitor, without being influenced by fluctuations in the AC power supply when charging the DC capacitor in the DC link and changes in time constants of the charging circuit and the discharging circuit. However, the capacitance of a DC capacitor typically has a dispersion of several tens % with respect to the nominal value; therefore, there is a fundamental problem that it is not possible to obtain an accurate determination result by a method for performing a life determination simply by comparing the capacitance of the DC capacitor with a certain reference capacitance. Further, when the capacitance of the DC capacitor is changed due to addition or removal of a device, replacement of a device or the like based on a system change, too, it is not possible to obtain an accurate determination result by a method for performing life determination by comparing the capacitance of the DC capacitor with a certain reference capacitance, and there is a problem that the reference capacitance to be used for life determination must be changed if it is attempted to obtain an accurate determination result.
FIG. 10 is a schematic view for explaining about a life determination when a device equipped with a DC capacitor is newly added in the invention described in Japanese Unexamined Patent Publication No. 2000-152643. When a device 120 equipped with a DC capacitor 105-1 is newly added to the circuit illustrated in FIG. 10, and on the assumption that the capacitance of the DC capacitor 105 is C1 and the capacitance of the DC capacitor 105-1 is C2, the combined capacitance “C1+C2” of the DC capacitor 105 and the DC capacitor 105-1 can be obtained by equation (2) as follows:C1+C2=∫Idt÷V  (2)
However, only from the combined capacitance “C1+C2” obtained from equation (2), it is not possible to determine to what extent the capacitance is decreased from the initial state. It may be considered to add the nominal value for the DC capacitor 105-1 to the so-far reference capacitance used to determine the life of the DC capacitor 105 and perform life determination with the resultant capacitance as a new reference capacitance, but since the capacitance of the DC capacitor has a dispersion of several tens % with respect to the nominal value as described above, the nominal value after the addition also includes a dispersion, and therefore it is not possible to achieve an accurate life determination.